The sinking of the Lusitania was one of the reasons the US joined the war. The US was trying its best to stay out of the war and avoid sending troops as long as it could. So they decided to send resources to their allies. They sent resources like bullets, weapons, etc by boat. One day, the US had a boat carrying people from the US to Britain. A German U-Boat encountered the Lusitania. They assumed it was a cargo boat sending supplies to Britain, so they shot it with torpedos. This ended up killing around 1,200 people and leaving 767 people stranded on lifeboats. This made them want to join the war because a lot of their civilians died for no reason, and a lot of families went through heart ships they didn't have to go through